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Worth is right, but all of that is beet leafminer damage. Most people are familiar with the meandering little trails left by leafminers on other plants but beet leafminers cause a lot more damage to members of the beet family, the damaged tissue is mushy which makes you think it may be a fungal infection. It's not a big deal for me, I've had that damage on a few leaves in the past, but no big infestation. If you can catch the eggs on the bottom of a leaf before the larva hatches you avoid the damage and stop a new generation of flies from coming back and laying more eggs.
If that's the only problem you have with Swiss Chard, count your blessings because there are far worse things I've heard.

Yes, leaf miners. I get those on beets and spinach but not usually on Swiss chard. I just break off the infected leaves and throw then on the ground to burn up in the hot sun so that the larvae can't hatch into flies and complete their life cycle.

I was doing that to my beets for the past 2 weeks and now all of the leaves are clear with no more damage. I don't usually have a real big problem with them.